The present invention generally relates to electromagnetic interference filters, and more particularly to an improved electromagnetic interference filter having a solder-in encapsulated filter construction.
Electromagnetic interference (EMI) is generally any undesirable electromagnetic emission or any electrical or electronic disturbance which causes an undesirable response, malfunctioning or degradation in the performance of electrical equipment. EMI propagates through conduction over signal and power lines, and through radiation in free space.
EMI filters are often used to attenuate electromagnetic interference. Generally speaking, an EMI filter is a passive electronic device used to suppress conducted interference present on a power or signal line. The EMI filter may be used to suppress the interference generated by the device itself, as well as to suppress the interference generated by other equipment to improve the immunity of a device to the EMI signals present within its electromagnetic environment.
There are a wide variety of input, output, and signal line EMI filters designed for ordinary commercial applications, and for military, aerospace, space and other high reliability industrial and commercial applications. Generally speaking, high reliability EMI filters use components that have an established reliability or failure rate, and the final filter assembly is screened using various military, environmental, stress and other applicable tests and standards. Commercial EMI filters generally use standard off-the-shelf components and are not subject to the testing described above.
One of the most common types of EMI filters for ordinary commercial applications has a tubular capacitor filter construction. In this construction, soldering or conductive epoxy is used to attach a tubular capacitor 1 to a filter housing 2, as seen in FIG. 1. Although relatively simple to manufacture, certain disadvantages of tubular capacitors make this type of filter relatively unreliable. For example, microcracks are typically induced in the tubular capacitors during the assembly and testing process, and lead to dead shorts in the filter. Additionally, tubular capacitors tend to have a high degree of porosity resulting from the relatively small volume to thickness ratio of the capacitors, which has the effect of lowering the dielectric breakdown voltage of the capacitor. Moreover, mechanical torque during the installation process can cause cracks in tubular capacitors leading to dielectric breakdown, and electrical environment testing such as thermal shock and burn-in can cause thermal fractures in the capacitors which also lead to dielectric breakdown. As a result of these disadvantages of tubular capacitors, this type of filter is generally not used in high reliability applications.
For high reliability applications, an EMI filter having a discoidal capacitor filter construction is often used in place of the tubular capacitor filter construction discussed above. As seen in FIG. 2, in a discoidal capacitor filter construction, a pair of monolithic discoidal capacitors 3 are used in place of the tubular capacitor. As described above, soldering or conductive epoxy is used to attach the discoidal capacitors to the filter housing 4. Additionally, certain epoxies 5 can be utilized to encapsulate the capacitors, and any other internal components of the filter, to avoid damage during thermal shock and vibration.
The geometry of discoidal capacitors 3 makes this filter construction superior in many aspects in comparison to the tubular capacitor filter construction described above. For example, discoidal capacitors have a greater ability to withstand torque stress than tubular capacitors, and are also less prone to thermal stresses. Additionally, discoidal capacitors generally have a lesser degree of porosity than tubular capacitors, which improves breakdown voltage strength.
Despite these advantages, there are some disadvantages associated with the conventional discoidal capacitor filter construction described above. For example, there is significant difficulty associated with the soldering process during which the discoidal capacitors are attached to the filter housing. Although less porous than the tubular capacitors, the discoidal capacitors still exhibit some degree of porosity on their surfaces. This porosity makes the discoidal capacitors susceptible to flux contamination, as some of the flux present during the soldering process is captured by the capacitors. Additionally, contamination may occur in the reflowed solder and in the filter housing. Although there are various cleaners available for removing flux from the filter, the discoidal capacitor filter construction can never be completely washed clean. The presence of flux that cannot be completely removed compromises the electrical properties of the filter, such as the insulation resistance of the filter. Since there are two discoidal capacitors exposed to the soldering process, there is a strong likelihood of flux contamination in the conventional discoidal capacitor filter construction.
Another problem associated with the conventional discoidal capacitor filter construction is the possibility of capturing moisture in the filter. Since the filters are epoxy encapsulated, it is impossible to achieve a hermetic seal, and thus there is the potential for moisture being captured in the filter during the assembly and testing process. The filters are typically designed for an operating temperature anywhere in the range of xe2x88x9255 degrees Celsius to 125 degrees Celsius. The filters are often put through testing across this temperature range which can draw moisture into the filter or exacerbate problems related to any moisture already captured in the filter, affecting the electrical properties of filter.
As a result of these problems and disadvantages associated with the conventional discoidal capacitor filter construction, the yield per lot after assembly and testing is significantly low, and not acceptable for many high reliability applications. Consequently, a need exists for an improved EMI filter construction.
The present invention, therefore, provides an improved EMI filter construction to minimize the disadvantages associated with the prior art filter constructions. In a presently preferred embodiment, the present invention provides a solder-in filter construction, where a pair of solder-in filters are assembled together outside of a filter housing, and then inserted into and soldered in place inside the filter housing. Each solder-in filter comprises a capacitor encapsulated within and attached to a metal casing. Additionally, each solder-in filter includes a glass hermetic seal on one end of its metal casing.
In an alternate embodiment, a single solder-in filter is used in combination with an exposed discoidal capacitor. The exposed discoidal capacitor is used in place of one of the solder-in filters because the size of certain filter housings is such that presently available solder-in filters will not fit into the neck of such housing. Although this process utilizes an exposed discoidal capacitor, the risk factor associated with flux contamination and other problems associated with such a construction is reduced by 50% when compared to the conventional discoidal capacitor construction, as a result of the elimination of one of the exposed discoidal capacitors.